text: Isotopes are different forms of elements; they have a different number of neutrons in the nucleus, meaning they have very similar chemical properties, but different mass.  The weight difference means that some isotopes are discriminated against in chemical processes – for example, plants find it easier to incorporate the lighter C than heavy C. Other isotopes are only produced as a result of the radioactive decay of other elements, such as Sr, the daughter isotope of Rb.  Rb, and therefore Sr, is common in the crust, so abundance of Sr in a sample of sediment (relative to Sr) is related to the amount of sediment which originated in the crust, as opposed to from the oceans.
  The ratios of three major isotopes, Sr / Sr, S / S and C / C, undergo dramatic fluctuations around the beginning of the Cambrian.